New Theory Links Moon's Current Orbit to Its Formation Via a Giant Impact

BOULDER, Colo., Feb 16, 2000 -- The mysterious tilt of the moon's orbit
is probably a natural consequence of the moon's formation from a giant
collision with early Earth, according to a new study by scientists at
Southwest Research Institute (SwRI).

The moon's orbit can be traced backwards in time to reveal that when the
moon formed near the Earth, its orbit was inclined by approximately 10
degrees relative to the Earth's equator. Most other planetary satellites in
the solar system have orbital inclinations smaller than 1 or 2 degrees. The
cause of the moon's large orbital tilt has long been a mystery.

"The inclination problem had been one of the last remaining obstacles for
the impact hypothesis of moon formation," says SwRI Institute Scientist
Dr. William R. Ward. The widely favored "giant impact theory" proposes
that a Mars-sized body collided with Earth 4.5 billion years ago, creating
a hot disk of debris from which the moon accumulated. Previous models of
the moon's formation from such a disk predict that the lunar orbit should
have been nearly aligned with the Earth's equator, with only about a 1
degree tilt.

The new theory, published in the Feb. 17 issue of Nature, proposes that the
moon acquired its large tilt soon after it formed because of a gravitational
interaction with debris left over from the impact event. Modeling results
presented in the paper, authored by Ward and SwRI planetary scientist Dr.
Robin M. Canup, show that the moon could have acquired its 10 degree tilt
as a consequence of the moon-forming impact.

To yield a lunar-sized moon, the giant impact must place about two lunar
masses of material into an Earth-orbiting disk, according to Canup. In the
model, debris particles in the inner regions of such a disk are prevented
from coalescing by Earth's gravity, which tends to pull objects apart.
Instead, the moon rapidly coalesces at the outer edge of the debris disk,
at a distance of about 14,000 miles from the Earth. "The newly formed
moon would have likely co-existed for some time with an inner disk of
gas and debris left over from the impact," says Canup.

After the moon coalesced, its gravity would generate waves in the inner
disk. The gravitational interaction of the moon with these waves would,
in turn, modify the lunar orbit. The waves are launched at certain locations
in the disk where the motions of disk particles are in resonance with the
motion of the moon.

The waves generated at one such resonance -- where the orbital period of
the moon is approximately three times that of the disk particles -- are
called "bending waves," which corrugate the surface of the disk. The
gravitational attraction between the moon and these rippled waves in
the disk then acts to amplify the tilt of the moon's orbit.

Ward and Canup's model simulated the interaction of the moon and the
inner debris disk, assuming that the moon formed in an orbit with only a
1 degree tilt. They found that the interaction of the moon with the bending
waves it generates in such a disk can amplify the lunar inclination to
values as high as 15 degrees before the disk dissipates. The required tilt
of about 10 degrees can be achieved if the disk contained at least 25 to 50
percent of a lunar mass and persisted from decades to as long as a century.
These values are consistent with those predicted by other models of the
impact event.

"This theory explains the moon's anomalous orbital tilt as a natural
consequence of its formation from a giant impact event," says Ward.
"Rather than producing conflicting evidence, the lunar inclination may now
represent an additional corroboration of the impact event," agrees Canup.

SwRI is an independent, nonprofit, applied research and development
organization based in San Antonio, Texas, with more than 2,700 employees
and an annual research volume of more than $300 million.